1. Field of the Invention
The present invention relates to a charging apparatus for charging a secondary battery such as a nickel-cadmium battery and a nickel-hydrogen battery.
2. Description of the Prior Art
Since secondary batteries used for portable electronic apparatuses are rechargeable, several charging apparatuses have been proposed for charging them. The voltage of a charged secondary battery decreases as the battery is used. It is known that if recharging before the battery is completely discharged is repeated several times, because of the memory function of the secondary battery, the battery cannot be used at voltages lower than the battery voltage of just before the recharging.
For example, referring to the characteristic of FIG. 3 in which the axis of abscissas represents the time and the axis of ordinate represents the battery voltage, if the use of the battery is stopped at A and the battery is recharged there, the battery cannot be used in the right voltage range exceeding the voltage at A. This means that the battery cannot be used in the voltage range between 1.2 V and 1.0 V on the right side of A (1.2 V) while the usable voltage range of one secondary battery is between 1.6 V and 1.0 V.
To solve this problem, when the battery is recharged at A, it is typical that the battery voltage is decreased to 1 V by discharging before the battery is recharged (the voltage may be decreased every time the battery is recharged or every several times). Thus, to recharge the secondary battery, it is necessary to discharge it before recharging it.
Therefore, the charging apparatus is provided with a discharging mechanism in addition to a charging mechanism. The charging apparatus typically has a controller to control the charging and discharging. As the power source for the controller, a DC (direct current) power source from an AC (alternating current) adapter is used. When the AC adapter is not connected to the commercial AC power source, since no DC power source from the AC adapter is present, the charging and discharging cannot be controlled.
In the case of the charging, the controller is supplied with power since the AC adapter is always connected to the commercial AC power source. However, in the case of the discharging, since it is unnecessary to supply power from the commercial AC power source through the AC adapter, the AC adapter is usually disconnected from the commercial AC power source. Therefore, the controller is deactivated so that the discharging cannot be controlled. To control the discharging, the AC adapter is necessarily connected to the commercial AC power source. In other words, discharging cannot be performed without the commercial AC power source.
In charging, since charging can be performed after discharging is performed, there may not be so frequent occasions when it is necessary to independently perform discharging. However, when it is necessary to independently perform discharging, in the conventional apparatus, discharging cannot be performed without the power from the AC adapter. The secondary battery may be discharged by a mechanical switch even if there is no power from the AC adapter and the controller is deactivated. However, this requires an extra operation, and in the case of the secondary battery, since it is dangerous to discharge the battery to a voltage below a predetermined lower limit, it is preferable to perform discharging under control of the controller.